Go for the 10-spd. I have a 2024 3/4 ton and 1 ton dually. We are now at over 100k miles between the two. The 3/4 ton doesn't tow as much and I have had zero issues. The 1 ton tows a couple times a week, 10-16k depending on the machine I am moving. Both trucks are used for construction and also spend time at the farm, have been great. Had an issue with the 1 ton and GM replaced the valve body in the transmission, it was down for 3 weeks which sucked, but it was all covered. It tows great and I really like the 6.6 L8T. Maintenance has been easy, I do all oil and fluid changes myself. The front end is greasible, so make sure you keep everything lubed up.
On our 2018 3/4 ton with the 6.0 I just had to use our unlimited powertrain warranty. I thought the warranty would never actually pay, but once they saw the engine was spotless inside at 180k and the only issue was a failed camshaft bearing, they paid 12k for a new engine. Always thought those warranties were a scam, but if you are diligent about maintenance and keep track of your records, they are great. Many of the oil changes were self performed, but I had all the receipts from buying the oil and filters, and had a log with the mileage I changed it at.
Alright so this one ended up being a different TSB that was not related to the transmission at all. It was the O2 sensor wire that got too close to the exhaust and melted. I guess the 5v reference wire melted, shorted out and caused about 200 codes to populate and the truck to go into limp mode.
https://static.nhtsa.gov/odi/tsbs/2024/MC-10253570-0001.pdf
Would the best option be to use a local dealer? I'll be flying into town, most dealers I've called want to charge to do a used car inspection. Can get expensive if I'm comparing 3+ trucks on the same day, not to mention scheduling.
All I see on CARFAX reports is "Engine/powertrain checked" or "Emissions system serviced" and on a few, "Transmission replaced" or "Valve body replaced" which is concerning. A lot of "Transmission Replaced" prior to 140k so I guess I'll have to plan for some extra cash in case of an emergency.
Is it normal for the 10L80 to downshift hard and flare when coming to a stop? I've now experienced it enough (along with some rear end noise when test driving) that I don't know what's a normal 10L80 in non-sport or Tow-Haul mode. Some trucks downshifted cleanly, others lurched and jerked as the RPMs jumped around even in Normal mode.
I'd go right back to the dealer. If I didn't get satisfaction there, I'd go to a different one. If not there, I'd contact GM. It's under warranty. It's their issue to fix.
A lot of people also forget the old 6spd was pretty good at the torque converter snout wearing into the front of the trans case. Pretty common issue. 6spd was not a perfect trans by any means.
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